And the engines needed to be throttled down soon after launch to avoid exceeding max Q or max structural dynamic pressure loads on the STS.
To do this, they took the shuttle main engines down to 67% thrust in the lower atmosphere and then back up to 104% when the atmospheric density reduced sufficiently.
The solid boosters were more sophisticated than I imagined. They each had an APU and vectored thrust nozzle.
Have a look at this superb footage of a launch. The Shuttle main engines start first - to make sure they are all healthy - before the boosters are started. You can see (and hear) the whole STS angle over with the force, and then the boosters are lit.
Awesome !!
https://m.youtube.com/watch?v=Lq_shHu4lAs